on the HDD question. would you put your drive in water? no, because water would get into it. it's not strictly airtight.
water molecules are quite small. They're one of the 3 simplest molecules in the universe (the others being H
2 and O2)
Anyway, you wouldn't put the PC in water either.
You could, however, put it in non-conductive oil. which is what they did here.
Not only is the oil they used non-conductive, but the oil molecules generally are much larger than water molecules.
So while water may be able to enter filters used by hard drives, I highly doubt that oil could.
Though I would expect that some hard drives do not even use a fine filter, but rather, a flexible material that will flex in and out depending on the pressure difference, to balance it out somewhat.
that's one of my questions : how long will it last before the oil reaches ambient temps and is basically useless?
The oil would still be far from useless even if it were at a higher temperature than air.
It has a much larger heat capacity than air does, and will much more readily absorb heat than air would.
NOW THAT is an awesome idea 
